The choice of tail rotor diameter will be influenced by the following considerations:
Advantages of a Large Diameter Advantages of a Small Diameter
Low power required in hover Low tail rotor and drive system weight High directional control power Helps solve perpetual tail heavy problem High stability in forward flight Low hub drag
Although these conflicting considerations would seem to leave every designer on his own, a study of existing helicopters reported in reference 10.10 indicates a remarkably good relationship for the ratio of tail rotor diameter to main rotor diameter as a function of the main rotor disc loading. Figure 10.8 shows that small tail rotors are used with low main rotor disc loadings and big tail rotors are used with high disc loadings. There are probably three reasons for this:
1. With a high disc loading, the main rotor is requiring so much power that saving power with a big tail rotor is attractive.
2. With a low main rotor disc loading, the tail boom is very long and the difficulty of balancing the helicopter with the center of gravity near the main rotor is made easier by using a small tail rotor.
3. When looking at other helicopters, the observed trend becomes aesthetically pleasing.
The trend in Figure 10.8 is expressed approximately by the equation:
DT _ 1
DM ~ 7.15 — .27 D. L.M
It may be significant that both the Hughes AH-64 and the Bell OH-58 as originally designed had directional control problems that were made less severe by increasing the diameter of the tail rotor to make it closer to the trend line.
FIGURE 10.8 Tail Rotor Diameter Sizing Trend
Source: Wiesner & Kohler, “Tail Rotor Design Guide,” USAAMRDL TR 73-99, 1973.